The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material capable of forming images with high density at high speed recording.
In general, a conventional thermosensitive recording material comprises a support material made of, for example, paper or a film, and a thermosensitive coloring layer formed on the support material, which thermosensitive coloring layer comprises as the main components, a colorless or light colored dye such as leuco dye, and a color developer such as a phenolic material which is capable of inducing color formation in the colorless or light colored dye upon application of heat thereto, for example, through a thermal head of a thermal printer. This thermal recording is attained by a chemical reaction between the colorless or light colored dye and the color developer upon application of heat thereto. As compared with other conventional recording materials, the above described type thermosensitive recording materials have advantages in that recording can be performed without requiring complicated steps such as development and image fixing; therefore, such recording is done speedly, quietly and at a low cost. Because of these advantages, the thermosensitive recording materials are widely used, for instance, for use with computers, facsimile apparatus, telex, medical measurement instruments and other measuring instruments in a variety of fields.
In accordance with the recent general demand for high speed recording, there is a great demand for a thermosensitive recording material that is suitable for use in high speed recording, in particular, for use with the above-mentioned fields.
Conventionally, thermosensitive recording materials for use in high-speed recording have been proposed, for instance, in Japanese Laid-Open Patent Application No. 53-39139, Japanese Laid-Open Patent Application No. 53-26139, Japanese Laid-Open Patent Application No. 53-5636, and Japanese Laid-Open Patent Application No. 53-11036. In those thermosensitive recording materials, thermo-fusible materials with a low melting point, such as a variety of waxes, fatty acid amides, alkylated biphenyls, substituted biphenyl alkanes, coumarinic acid derivatives, biphenyl amines, are added to the thermosensitive coloring layer as a sensitizer or as an agent for reducing the melting point of the thermosensitive coloring layer.
In the case where the above described sensitizer agents are employed, it is necessary that the sensitizer agent be melted prior to the coloring reaction. Therefore, in the high-speed recording by application of a small amount of heat energy in an extremely short time, for instance, by heat-application impulses, a sufficiently high thermal response for practical use cannot be obtained. In addition to the above problem, in the above case, since the thermo-fusible materials are melted within the thermosensitive coloring layer, the accumulation of the melted thermo-fusible materials on a thermal head during an image recording process, trailing of the printed images and formation of ghost images are apt to occur. Furthermore, fogging is apt to occur in the background of thermosensitive recording material during storage at high temperatures and high humidities. As a result, the contrast of the recorded images decreases during storage.
Therefore, it is more preferable not to use the above described sensitizer agents or melting-point reducing agents in the thermosensitive recording materials for use in high speed recording.
Conventional colorless or light-colored dyes which are colored upon application of heat thereto for use in the thermosensitive recording materials, for example, dyes having lactone rings, lactam rings or spiro-pyran rings, usually have the melting points in the range of 160.degree. C. to 240.degree. C. Due to such high melting points of the colorless or light colored dyes, when the above described sensitizer dyes or melting points reducing agents are not employed, phenolic materials with particularly low melting points are used in combination as a color developer for inducing color formation in the above-mentioned colorless or light-colored dyes, in order to obtain thermal recording materials with high thermal sensitivities suitable for use in high-speed recording.
Phenolic color developers are described in many references including Japanese Patent Publication No. 45-14039. Among a large number of phenolic color developers, 2,2-bis(4'-hydroxyphenyl)propane (or Bisphenol A, m.p. 156.degree.-158.degree. C.) is preferably used, because of the stability of the quality as the color developer, and the reasonable price and availability. Bisphenol A, however, has the shortcoming that its color inducement temperature is relatively high.
In contrast to this, in the case where phenolic materials with low melting points, for instance, monohydric phenols, such as 4-t-butylphenol (m.p. 94.degree.-99.degree. C.), .alpha.-naphthol (m.p. 95.degree.-96.degree. C.), and .beta.-naphthol (m.p. 119.degree.-122.degree. C.) are employed, the preservability and stability of the thermal recording materials are so poor that the background thereof is discolored with time at room temperature during storage. Further, thermal recording materials containing such monophenols therein have a characteristics phenolic odor and are not suitable for practical use.
In Japanese Patent Publication No. 54-12819 and Japanese Patent Laid-Open Application No. 55-27253, 2,2-bis(4'-hydroxyl phenyl)-n-hexane (m.p. 99.degree.-103.degree. C.) and 1,1-bis(4'-hydroxyl phenyl)alkane (the alkane having 3 to 13 carbon atoms) are respectively disclosed as having low melting points and being capable of yielding images in a stable manner when they are employed in thermosensitive recording materials. However, these materials are difficult to synthesize and cannot be obtained easily.
In Japanese Laid-Open Publication No. 56-144193, it is described that a lower alkyl ester and a benzyl ester of p-hydroxybenzoic acid can be easily synthesized, and, by use of the esters, thermosensitive recording materials with high sensitivity can be prepared. However, the thermosensitive recording materials using the esters have the shortcomings that the colored portions easily discolor and the phenolic materials are separated in the developed image areas in the form of white crystals or powders.
As described above, there are known no thermosensitive recording materials which yield images with high density, high sharpness and high stability in high-speed recording, using a color developer that can be employed for industrial and practical use.